Afferent renal nerves have been implicated in the control of arterial pressure, fluid balance and the intense pain due to ureteral obstruction. Electrical stimulation of the renal nerve causes reflex changes in arterial pressure and sympathetic activity. Renal denervation has been shown to attenuate the development of many models of experimental hypertension in rats. Mechanical or chemical stimuli activate specific populations of afferent fibers in the renal nerve with corresponding changes in efferent sympathetic activity. The central projections of modality-specific renal afferent information, the pathways it follows to sites mediating sympathetic and behavioral responses and the relative excitability of these projections are unknown. This project is designed to identify the spinal and supraspinal projections of renal afferents, the modality of specific projections and the nature and degree to which descending or segmental inputs modify renal afferent information. Using electrophysiological techniques, the location and properties of cells in the gray matter of the spinal cord responding to renal nerve stimulation will be identified in the rat. The convergence of cutaneous input on these cells and its interaction with renal input will be examined with respect to the phenomenon of "referred pain" whereby pain of visceral origin is interpreted as being from somatic fields. The pathway and termination fields of ascending projections from these cells will be determined by antidromically activating cells from the cervical spinal cord and supraspinal sites. Responses of spinal cord cells to specific chemical and mechanical stimulation of the kidney will also be determined to identify sites responsive to specific stimulus modalities and to try to interpret their role in spinal and supraspinal reflexes. Finally, descending modulation of afferent renal input will be examined. Tonic modulation will be studied by comparing responses to renal nerve activation in intact and spinally-transected or spinally-blocked rats. Phasic modulation will be examined by identifying sites in the brain and spinal cord from which stimulation-induced facilitation or inhibition of responses to renal nerve activation can be elicited. These studies are designed to clarify the functional and anatomical projections of renal afferent fibers in order to better understand their role in normal and pathological states.